A common practice is to make forsterite products from olivinites, which are naturally occurring magnesium silicates rock containing essentially olivine which is a solid solution of forsterite (2MgO.SiO.sub.2) and fayalite (2FeO.SiO.sub.2). In such cases, finely divided magnesia can be added for the purpose of converting any more siliceous magnesian minerals carried in the olivinite to the more highly refractory forsterite. Nevertheless, it should be appreciated that the iron oxide in olivine is present in the form of fayalite and cannot be removed from the olivine. The disadvantage of fayalite is that it acts as a flux and thus decreases the resistance to heat of refractories made from olivine.
Forsterite products have been made also from other natural magnesium silicates of high refractoriness such as dunites which are essentially composed of olivine and small amounts of serpentine (3MgO.2SiO.sub.2.2H.sub.2 O) and from other natural magnesium silicates of relatively low refractoriness, for example such hydrothermal alteration of magnesium silicates as serpentinites, talcs and steatites, by the addition of sufficient magnesia to form forsterite from these more siliceous magnesium silicates and hydrosilicates.
Extended use of forsterite brick made from such natural magnesium silicates has shown that under the conditions encountered in many high temperature applications they give markedly successful and satisfactory performance. However, it has been found that such forsterite brick undergo progressive deterioration when they are exposed at elevated temperatures alternately to oxidizing and reducing conditions, exemplified by the cyclic operation of glass tank regenerators. More in detail, these bricks under such cyclic conditions undergo progressive loss of bond, and ultimately they disintegrate. Accompanying those phenomena is an expansion that may amount to as much as 5 percent, with the possibility of creating undesirable stress conditions to the structure.
It is also known that the iron oxide content of bricks made from natural olivines is a limiting factor, and as a trend toward higher regenerator temperatures develops, and often increasing pollution from the furnace gases with batch constituents occurs, it has become increasingly apparent that the natural magnesium silicate rocks have reached the limit of their usefulness because of inherent iron content.
After appreciating the disadvantage of too high an iron content in natural olivines or other natural magnesium silicate rocks the industry attempted to prepare improved forsterite refractory bricks by the use of synthetic forsterite made by firing a mixture of high purity silica with high purity synthetic magnesia. Nevertheless, though this procedure is highly effective it requires the expensive pure magnesia and pure silica. Dead burned magnesia is obtained at temperature above 1600.degree. C., and after cooling it is ground to a proper grain size before it can be used for the manufacture of synthetic forsterite brick. Obviously all the operations contribute to the high cost of the final product.
Finally, others have appreciated that the presence of iron eventually caused certain deficiencies in refractory bricks made from naturally occurring magnesium silicates and therefore attempted to decrease the iron content by combining a substantially iron-free periclase with a natural magnesium silicate whereby the total iron content was reduced to less than 3%. It has been noted that such refractory bricks present certain disadvantages because of the disproportionate ratio of magnesium oxide to silicon oxide while having a very low iron oxide content.
It is also known that serpentinite, more particularly chrysotile asbestos tailings contains the constituents of olivine, that is magnesium oxide, silica oxide and ferric oxide though in different proportions. It is also known that there is an enormous amount of chrysotile asbestos tailings and residues in countries where asbestos mining is carried to a large extent so that it would appear that great advantages could be derived if a satisfactory method could be found to modify or convert the plentiful asbestos tailings to an olivine-like material, more particularly to an harzburgite and to provide a magnesium silicate having a very low iron oxide content and a chemical composition which approximates the composition of forsterite as far as the ratio of silica to magnesia is concerned.
Accordingly, in view of the exceedingly large amounts of chrysotile asbestos tailings and residues presently available in many parts of the world, and in view of the low commercial value of these tailings, it would appear highly desirable if a process could be used to convert these tailings to harzburgite materials suitable for the manufacture of refractory bodies.